The present application relates to aircraft power generation systems, and specifically to synchronous machine starter/generators.
It is common in the aerospace industry to use a three phase synchronous machine to generate electrical power in an aircraft and to use the same three phase synchronous machine to provide a motive force for starting a turbine engine. In order for the synchronous machine to provide a motive force to a turbine engine, three phase power is provided to the synchronous machine from a three phase starter generator through a set of phase connections. The three phase power causes the synchronous machine's rotor to begin turning. The rotor is mechanically connected to a turbine engine such that when the rotor begins turning, the turbine is forced to turn as well. Once the turbine engine has started and is in a state where it can operate independent of the synchronous machine, the synchronous machine switches to a “generate mode” where a DC exciter field driver provides two DC phases with a switching voltage in order to generate an excitation field current needed for the synchronous machine to generate three phase power. In generate mode, the rotation of the turbine engine causes the synchronous machine to rotate, which in turn causes the synchronous machine to generate electrical energy according to known techniques.
In order to reduce weight and components, DC switching voltages are typically transmitted from the exciter field driver to the synchronous machine over two of the phase lines used by the starter generator during the start mode, and the third phase line is deactivated. This configuration results in voltage spikes in the deactivated phase each time the switching DC voltage switches from positive to negative (or vice versa), and can result in a voltage buildup within a generator control unit under certain conditions. Large voltage spikes can lead to breakdown of connectors/wiring and cause arcing. The arcing, in turn, can lead to a shorted rotating diode in the synchronous machine. If a shorted rotating diode condition is caused by other reasons, then in addition to extremely large voltage spikes in deactivated phase, there will be a rapid voltage buildup within the generator control unit.
Currently, methods are known for detecting shorted rotating diode conditions, however the known methods require a long response time. Rapid voltage buildup in the generator control unit can decrease the possible time to respond to a shorted rotating diode condition below the response times of the known methods. No methods are currently known in the art for reducing the rate of voltage buildup within a generator control unit.
The same or similar issues can arise in non-aerospace applications, including land based turbine generators, when an exciter field driver is used in a similar configuration with a synchronous generator.